Enhanced capability for user equipment (UE) positioning is widely seen as a distinguishing and value increasing feature of 3G mobile communications networks such as UMTS W-CDMA FDD or TDD or CDMA2000.
An example for a simple yet effective user equipment (UE) positioning method that is conventionally used in 2G mobile communications systems is the “Cell-ID method”, where a UE's position is known on a cell level. The network knows the base station with which the UE has a radio link and can therefore assume that the UE is somewhere within the coverage area of that base station's cell.
Drawbacks of the Cell-ID method are that it is too coarse for macro and large micro-cell deployments and unreliable for small micro and pico-cell deployments.
Another example of a simple location technique is estimation of radial distance per round-trip-time based upon time of arrival of a UE's uplink (UL) transmission at the base station.
An example of a UE positioning method that has been standardized for 3G systems is the observed time difference of arrival (OTDOA) method. As shown in FIG. 1, a UE measures the time of arrival of the downlink (DL) signals from the base stations of several surrounding cells and reports these measurements (or quantities derived from these) back to the network. The location entity in the network then applies basic triangulation techniques based on distances d0, d1, d2 in order to estimate the UE's position. For example, UE measures the time difference of arrival between serving base station 101 and neighboring base station 102, yielding an elliptic band B11 with respect to where UE's position can be. The UE then measures the time difference of arrival for base station 101 and neighboring base station 103, to yield another elliptic band B12 with respect to the UE's possible position. Now, the UE position must necessarily be at the intersection of bands B11 and B12. Each band has an inherent measurement error margin em. For additional precision, further OTDOA measurements of pairings to other nearby base stations can be used to fine tune the estimate for the point of intersection. The precision and reliability of all OTDOA-based methods increase with increased availability of measurement data, especially when the UE can measure more than just 2 or 3 surrounding base stations. However, a limitation of this method is that the serving base station transmission has a blocking effect on the signals from the surrounding base stations.
FIG. 2 shows an example of how OTDOA performance can be enhanced to combat the blocking effect. Serving base station 201 fully or partially ceases its DL radio transmission intermittently in order to give each UE present in its cell an opportunity for measuring DL parameters of transmitting base stations 202-207 in surrounding cells.
A common feature for OTDOA-based methods is that a position estimation is based upon a triangulation technique where measurements are performed at the UE location on DL signals of surrounding base stations. Positioning accuracy and reliability limitations of OTDOA-based methods arise in small cell sizes and multipath environments.
Another group of UE positioning methods that has been standardized for 3G systems is based upon GPS technology that uses satellite assisted positioning. In GPS, the communication handset receives data transmitted continuously from the 24 NAVSTAR satellites. Each satellite transmits data indicating the satellite's identity, the location of the satellite and the time the message was sent. The handset compares the time each signal was received with the time it was sent to determine the distance to each satellite. Using the determined distances between the satellites and the handset along with the location of each satellite, the handset can triangulate its location and provide the information to a communication base station. However, the incorporation of a GPS within a subscriber unit increases its cost.
Accordingly, it is desirable to have alternate mobile user positioning schemes.